Upstream: undo feature id sharding about carmen-cache HOT 3 OPEN

This looks very old and perhaps stale. /cc @miccolis - can this be closed?

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cc/ @apendleton

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Hm, old, but not really stale? Feature IDs are still truncated to 20 bits, we still don't enforce uniqueness, we still clumsily handle collisions, and assuming the API boundary between carmen-cache and carmen stays the same, we'd need to move to a new way of communicating grid data back and forth (that is, not integers anymore) in order to work around it. I think there are probably even bigger grand-vision questions around carmen-cache now than there were then, but at least some potential avenues forward would involve executing on the plan proposed here.

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